This invention relates to a microwave oven and, more particularly, to a control circuit for controlling a magnetron of a microwave oven, which has a large scale integrated circuit.
Recently, it has been known to provide a control system for a microwave oven, including a programmable digital logic control circuit which enables a user to program information into the digital logic control circuit. An example of such a control system is shown in Fosnough et al U.S. Pat. No. 4,011,428 issued Mar. 8, 1977, entitled "Microwave Oven Timed and Control Circuit". A further example of a control system for a magnetron related to the programmable digital logic control circuit is disclosed in Zushi et al U.S. Pat. No. 4,295,027 issued Oct. 13, 1981, entitled "Microwave Ovens with Programmable Control".
In the above teaching, the microwave oven has a control circuit which memorizes a time for cooking and a cooking temperature for the food being cooked based on information supplied by the user. If the cooking time has elapsed, or the food reaches the predetermined cooking temperature, the microwave oven stops operating.
Moreover, the control circuit of the microwave oven functions to control the heating output by a key button on the control panel. In this operating mode, the microwave oven operates to cook the food according to a predetermined output. Also the microwave oven has a display function for displaying the predetermined cooking time, temperature, and energy output level, and the remaining cooking time, and temperature of food during cooking using LED display means.
Microwave ovens without digital logic circuits are readily available and generally have a low purchase price. These ovens have a mechanical timer for cooking and a mechanical output controller for controlling the microwave energy output, as shown in U.S. Pat. No. 3,824,365, issued on July 16, 1974.
It is advantageous for the low-cost ovens to also have the digital control circuits in order to operate with the above-mentioned temperature and output control functions.
A conventional microwave oven of this type is shown in FIGS. 1, 2 and 3.
Referring to FIG. 1, a body 1 has a door 2 and a front panel 3 located on a front face of the body 1. The door 2 is pivotally hinged at one side of the body 1. The front panel 3 has a sliding knob 4, a temperature control knob 5, a timer knob 6, and a start button 7. The microwave output energy is set by the sliding knob 4. Reference numeral 8 is a cook indicator lamp. A heating chamber (not shown) is constructed in the body 1 adjacent the door 2. The microwave energy is provided from a magnetron into the heating chamber to heat food products placed therein. A temperature detector, such as a temperature-sensing probe, is provided in the heating chamber to detect the temperature of the food.
FIG. 2 illustrates a circuit configuration of the control system of the microwave oven including the digital logic circuit. The circuit configuration mainly comprises a commercial power source 10, a magnetron 20 and a control unit 30.
The commercial power source 10 is connected to a primary winding 12a of the high voltage transformer 12 through a fuse 14, a door switch 16, a triac 18, and a relay contact 19a of a relay 19b. A secondary winding 12b of the transformer 12 is connected to the magnetron 20. Reference numeral 22 is a door monitor switch which is opened when the door is closed.
The power source 10 is also connected to a primary winding 24a of a voltage transformer 24 through the fuse 14. A secondary winding 26b of the transformer 26 is connected to the control unit 30.
The control unit 30 employs food temperature control of food to be cooked. The control unit 30 is connected to a temperature-sensing probe 32, a mechanical timer motor 6M, the relay 19b, and the triac 18. The temperature-sensing probe 32 comprises a thermometer 32a, which is removably connected to the control unit 30 by a sliding contactor 32b positioned at the wall of the heating chamber. The mechanical timer motor 6M is set with the desired cooking time by turning the timer knob 6. Reference numeral 6a is a timer contactor of the timer motor 6M. Contactor 6a closes when a cooking time is set by means of timer knob 6. Contactor 6a opens in response to the timing out of timer motor 6M.
Reference numerals 4R and 5R show variable resistors which change the microwave energy output and the set cooking temperature of the food in response, respectively, to the sliding knob 4 and the temperature control knob 5.
The triac 18 is turned ON or OFF by a signal from the control unit 30 according to the variable resistor 4R. Contactor 7a is turned ON to start the cooking process in response to depression of the starting button 7.
FIG. 3 shows a more detailed schematic of the control unit and associated circuitry. An output control means 40 is made of a large scale integrated circuit 40. LSI circuit 40 may comprises a microcomputer, as, for example, NEC PD550C. The contactor 7a produces a starting signal A for the LSI circuit 40, if the starting button 7 is pushed. After setting up a cooking time, the contactor 6a produces a cooking time signal B for the LSI circuit 40.
The variable resistor 5R provides operational amplifier (op-amp) 42 with a temperature setting signal from the variable terminal thereof. The thermometer 32a provides op-amp 42 and op-amp 44 with a signal representative of the food temperature.
Additional resistors are utilized to regulate the signals, as shown in FIG. 3. Vcc and Vd indicate fixed DC voltages generated from circuitry including the transformer 24.
A temperature-detecting signal C is provided for the LSI circuit 40 from the op-amp 42. Probe signal D is provided for the LSI circuit 40 from the op-amp 44.
Relay coil 19b is connected with npn-type transistor 46 through a resistor. If transistor 46 is turned ON by the control signal E of the LSI circuit 40, the relay coil 19b actuates the relay contactor 19a to close the circuit.
Accordingly, the control unit 30 controls the output of the microwave energy by producing the control signal E for the relay 19 according to the signals A, B, C, and D.
However, the control unit 30 may not operate properly if the LSI circuit 40 malfunctions. In this case, the microwave oven may not turn off even though the cooking time has expired or the predetermined cooking temperature has been reached. In such cases, it is dangerous for the microwave oven to continue operation after expiration of the cooking time.